Stents are endoluminal devices that are inserted into body lumens and expanded in order to maintain the patency of the lumen. It is known for example, to use a stent to maintain the patency of an artery, a urethra, or a gastrointestinal organ. A temporary stent is left in the lumen for a predetermined period of time, while a permanent stent is intended to remain permanently in the body.
A stent is an elongated device that can exist in two conformations. In the small caliber conformation, the stent is inserted into the body and delivered to the lumen to be treated. Once correctly positioned in the lumen, the stent is deployed by being brought into a large caliber in which it applies radially outward forces against the inner wall of the lumen. The stent is constructed so as to be able to withstand radially inward forces applied to it by the lumen wall, so that the caliber of the stent is maintained after deployment in the lumen.
A stent may be formed, for example, from an elastic material that is unstrained when the stent is in the large caliber conformation. The stent is then mechanically constrained to bring it into the small caliber conformation. This stent may be maintained in the small caliber conformation by inserting it into a restraining sleeve. After positioning in the body, the restraining sleeve is removed. Due to the elastic properties of the stent, the stent spontaneously transforms into the large caliber conformation.
It is also known to form a stent from a material that becomes plastic when strained. The stent is formed in the small caliber conformation in which it is unstrained. A balloon is inserted into the lumen of the stent. The stent is then positioned in the body and the balloon is inflated. This expands the stent into the large caliber conformation by causing a plastic deformation of the stent material.
It is further known to form a stent from a shape memory alloy, such as Nitinol™. A shape memory alloy may exist in two states: a state in which it is super-elastic (the austenitic state) and a state in which it is soft (the martensitic state). The alloy in the austenitic state is formed into a stent in its large caliber conformation. The alloy is then brought into the martensitic state either by cooling the alloy or straining it. In the martensitic state, the alloy is deformed into the small caliber conformation in which it is delivered to the lumen to be treated. After positioning, the alloy is brought into the austenitic state by heating the alloy. In the austenitic state, the stent regains the large caliber conformation due to the shape-memory properties of the alloy.
It is further known to form a stent from a biostable or biodegradable elastic shape memory polymer. The shape memory capability of these polymers allows stents made of these materials to be inserted through small openings and then enlarging their caliber by an increase in temperature. The shape memory effect of polymers is a physical property exhibited best by amorphous polymers whose glass transition temperature is marginally higher than room temperature and whose transition from glass to rubber is particularly sharp. In this case, strain energy can be stored in the polymer by mechanical deformation (e.g. by stretching) followed by cooling. Recovery of the shape memory is exhibited upon reheating the material above the temperature to which it was cooled, allowing a return of the stretched polymer chains to more equilibrium, coiled structures.
In the vascular system a typical stent is about 1.2 times the vessel diameter to ensure appropriate anchoring without causing excessive pressure to the vessel wall. Due to the circular cross-section of the vessel lumen and the stent, after deployment, the entire stent is in contact with the entire surrounding vessel wall. If the wall of the stent is fenestrated, as it expands and presses all around against the vessel wall, some pressure damage to the vessel endothelium occurs. This damage induces a chain tissue reaction until the stent becomes covered with endothelium.
Benign Prostate Hyperplasia (BPH) is the most common tumor affecting human males. As the tumor grows in the prostate, it constricts the prostatic urethral lumen. This constriction reaches a point where the prostatic urethra becomes obstructed, and voiding of urine is inhibited.
There are several methods for treating BPH. The most common treatments include medication to reduce the pressure on the prostatic urethra or reduce the size of the prostate, and in more extreme cases, open or endoscopic surgery, to remove the tumor and widen the prostatic urethra. Surgical BPH treatments typically include open surgery, such as open prostatectomy, and endoscopic surgery such as Transurethral Prostatectomy (TURP), Transurethral Incision of the Prostate (TUIP), and Transurethral Laser or Radiofrequency (RF) vaporization of the prostate. Minimally invasive surgical procedures based on inducing thermal damage to the enlarged prostatic tissues have also been developed. After heating or freezing the prostatic tissue, a scarring process gradually occurs that shrinks the bulk of the tissue, relieving the obstruction.
In all surgical procedures for the treatment of BPH, a catheter is left indwelling in the urethra for a few days. After minimally invasive treatments, many patients often cannot void at all following catheter removal and they must therefore be recatheterized. This is typically due to the continuing oedematous swelling of the tissues. Discomfort can continue for the duration of the natural healing of the wound which may take several weeks. It is well documented that urethral catheters left indwelling more than 48 hours increase the risk of infection, in addition to the discomfort the catheter causes the patient. The infection occurs due to communication between the bladder and the outside world. Bacteria ascends from the urethral opening toward the bladder over the outer surface of the catheter or through its lumen. In such cases, insertion of a temporary sent instead of an indwelling catheter is usually more comfortable for the patient and minimizes the risk of urinary infection.
The use of a temporary stent having a round cross-section that does not conform to the shape of the lumen may cause discomfort and pain to the patient. Some temporary stents used for BPH treatment include an anchor that is connected to the stent by a wire. The stent is deployed in the prostatic urethra, with the anchor positioned in the bulbar urethra, the stent and anchor being connected be a wire through the voluntary urethral sphincter between the prostatic and bulbar urethra that controls the flow of urine from the urinary bladder. This sphincter is a formation of muscle tissue encircling the urethra, contraction of which occludes the urethra to prevent flow of urine from the bladder. Examples of such stents are the Prostakath and the Prostacoil. Other temporary stents, such as the Memokath and the Horizon stent have a bell-shaped end at its sphincteric part for anchoring. Temporary stents without cross-sphincteric parts are more prone to migration
It is also known to insert a permanent stent into the prostatic urethra for the treatment of BPH obstructions. Most permanent prostatic stents were originally designed for vascular use and were adopted for prostatic use by mainly changing the caliber and length of the basic design. From among the different vascular stents, the ones that have been used in the urinary tract are the balloon expandable Palmaz stent (marketed as the Titan); the self expanding Strecker, and the Wallstent. The use of the balloon-expandable Palmaz stent was abandoned because of its rigidity, the unacceptably high migration rates of the stent in the urethra, and its inability to become completely embedded into the wall. The braided Wallstent and the knitted Strecker stent have also been used in the urinary tract as permanent stents. However it was the Wallstent which became the most popular permanent stent in urology despite its limitations.
A fenestrated permanent prostatic stent, like a vascular stent, would be expected to react with the urethral tissues, resulting in its becoming embedded in the wall and being covered by a layer of epithelium. However, a prostatic urethral stent does not always become fully embedded in tissue. Milroy and Ng (K. J. Milroy E. J. G., Anatomical limitations of the prostatic urethra in using cylindrical stents. In: Stenting the Urinary System. Ed. D. Yachia. ISIS Medical Media, 1998. Chapter 42. Pages 319-321] studied the failure of permanent prostatic stents to become fully embedded in tissue. They performed 3-Dimensional ultrasonographic scans and found that in BPH the prostatic urethra is deformed by the enlarged prostate lobes and thus does not have a circular cross section. Because of the non-circular cross-section of the prostatic urethra, the tissue coverage of the stent in many cases is not complete. The uncovered, bare wires that remain in constant contact with urine become gradually covered by urinary salts and cause stone development and infection.
FIG. 1 shows schematically views of the lumen of the prostatic urethra in different individuals as might be observed, for example, by an endoscope inserted in the urethra. In FIG. 1a, a prostatic urethra is represented whose lumen 105 has a so-called “A” shaped cross section. In FIG. 1b, a prostatic urethra is represented whose lumen 110 has a so-called “J” shaped cross section. In FIG. 1c, a prostatic urethra is represented whose lumen 100 has a circular or, so-called “O” shaped, cross section.
FIG. 2 shows schematic views of the lumens shown in FIG. 1 after deployment of a stent 200. In FIGS. 2a and b, a stent 200 was deployed in the lumens 105 and 110, respectively. However, since the lumens 105 and 110 did not have an O shape prior to deployment of the stent 200, regions 210 of the outer surface of the stent 200 are not in contact with the wall 205 of the urethra. Under these anatomical conditions, some regions 210 of the permanent stent will not become covered by epithelium. In FIG. 2c, a stent 200 having a circular cross section was deployed in the lumen 100. In contrast to FIGS. 2a and 2b, since the lumen 100 had an O shape prior to deployment of the stent 205, the entire outer surface of the stent 200 is substantially in contact with the wall 205 of the urethra. The entire stent 200 in FIG. 2c can thus be expected to become covered by epithelium.